Field
Embodiments relate to the field of fuel cells. In particular, embodiments relate to conductive polymer layers to limit transfer of fuel reactants to catalysts in fuel cells.
Background Information
Fuel cells are electrochemical cells that convert chemical energy from a fuel reactant into electricity. Examples of fuel reactants include hydrogen, alkanols, alkanes, and other hydrocarbons. Fuel cells are different from conventional batteries in that fuel cells are open systems. Reactants including the fuel are introduced into the fuel cell, and reaction products, as well as any unused reactants, are removed from the fuel cell. In contrast, conventional batteries have a finite amount of stored energy. Once this finite amount of energy has been used up, the battery either needs to be discarded or recharged (e.g., plugged into an outlet). However, fuel cells can be replenished with additional reactant, which allows the fuel cells to achieve continuous operation for long run times. For these and other reasons, fuel cells are increasingly being considered for use in powering different types of electronic devices and systems.
FIG. 1 is a block diagram illustrating a simplified direct methanol fuel cell (DMFC) 100 and showing methanol (CH3OH) crossover. DMFCs are a known type of fuel cell where methanol (CH3OH) is used as the fuel. The DMFC includes anode 102, cathode 106, and polymer electrolyte membrane (PEM) 104. The PEM is coupled between the anode and the cathode. In DMFCs, the PEM is sometimes referred to as a proton exchange membrane.
During operation, half-reactions take place at each of anode 102 and cathode 106. The anode and cathode each typically include a catalyst to catalyze or accelerate the half-reactions. Methanol (CH3OH) and water (H2O) are introduced into the anode as reactants. A first half-reaction takes place at the anode, in which a molecule of methanol (CH3OH) reacts with a molecule of water (H2O) to produce a molecule of carbon dioxide (CO2), six protons (6H+), and six electrons (6e−) as products. The protons (H+) are transferred or exchanged across PEM 104 from the anode to the cathode as shown by the leftmost arrow in the PEM. Water (H2O) is also transferred or exchanged into the PEM to keep the PEM hydrated to enhance operation (e.g., proton transport), as shown by the middle arrow in the PEM. The electrons (e−) are conducted as electricity along a conductive path 108 from the anode to the cathode through an intervening external load 110 (e.g., a circuit, electronic device, etc.). Oxygen (O2) or a source of oxygen (e.g., air) is introduced into the cathode as a reactant. A second half-reaction takes place at the cathode, in which stoichiometrically 1.5 molecules of oxygen (O2) react with the six generated protons (6H+) and the six generated electrons (6e−) to produce three molecules of water (3H2O) as a reaction product.
One significant challenge faced in fuel cells in general, and in DMFCs in particular, is reactant crossover (e.g., methanol crossover). As shown by the rightmost arrow within the PEM, in methanol crossover, some of the methanol that is introduced into anode 102 as reactant is transferred, without reacting, from the anode to cathode 106 across PEM 104. In other words, un-reacted methanol crosses over from the anode to the cathode across the PEM. In DMFCs, the PEM is generally designed to allow some water (H2O) permeability or uptake in order to promote hydration of the PEM and provide good proton (H+) transport. However, methanol and water are relatively similar in size and hydrophilicity (i.e., affinity for water), and consequently the PEM is generally not sufficiently selective to allow the desired amount of water uptake without allowing methanol crossover.
Methanol crossover may lead to various potential problems. For one thing, the crossover methanol may poison the cathode catalyst, which is commonly platinum or a platinum containing material. As a result, greater amounts of the cathode catalyst are commonly deployed in order to provide allowance for the poisoning. However, this tends to increase the cost of manufacturing the fuel cell. For another thing, methanol crossover tends to decrease the operating voltage of the fuel cell and decrease the resulting power output of the fuel cell. The crossover methanol, after reaching the cathode catalyst, may react quickly with oxygen (i.e., oxidize) on the cathode catalyst, which may reduce the operating voltage of the fuel cell. These or other known problems may be expected when other types of reactants besides methanol, such as, for example, other alkanols, alkanes, or other hydrocarbons, unintentionally or undesirably crossover PEM 104 from one side to the other.
Accordingly, reducing reactant crossover in fuel cells may offer certain advantages.